The relentless drive to reduce feature size in microelectronics has continued now for several decades and feature sizes have shrunk in accordance with the prediction of Gordon Moore in 1975. While the repeal of Moore's Law has been anticipated for some time, we can still confidently predict that feature sizes will continue to shrink in the near future.
Using the shrinking-feature-size argument, it became clear in the early 1990's that it would be possible to put more than 3 or 4 charge coupled device (CCD) electrodes in a single pixel. Thus, the complimentary metal oxide (CMOS) active pixel sensor concept was born. The effective transistor count in most CMOS image sensors has hovered in the 3-4 transistor range, and if anything, is being reduced as pixel size is shrunk by shared readout techniques.
The underlying reason for pixel shrinkage is to keep sensor and optics costs as low as possible as pixel resolution grows. Camera size has recently become highly important in the rapidly expanding camera-phone marketplace. Concomitant with the miniaturization of camera components such as sensors and optics is the miniaturization of optical system components such as actuators for auto-focus and zoom in megapixel camera phones.
We are now in an interesting phase in the development of image sensors—for both CCDs and CMOS active pixel sensors. The physical dimensions of the pixel are becoming smaller than the diffraction limit of light at the wavelengths of interest. A perfect lens can only focus a point of light to a diffraction-limited spot, known as an Airy disk and the Airy disk is surrounded by higher order diffraction rings. The Airy disk diameter DA is given by the equation:DA=2.44λF#Where λ is the wavelength and F# is the F-number of the optical system.
For example, at 550 nm, and F-number of 2.8, the Airy disk diameter is 3.7 μm. Yet, pixel sizes in megapixel image sensors are at this size and smaller today. We refer to pixel sizes smaller than the 550 nm Airy disk diameter as sub-diffraction-limit (SDL) pixels.
Today it is readily possible to build a 6-T SRAM cell in less than 0.7 μm2 using 65 nm CMOS technology. Smaller device are being prototyped. However, significant issues exist for a 0.25 μm2 pixel, even though it might be tempting to make a 2 megapixel sensor with 1 mm diagonal using such small pixels. For example, the resolution of the sensor would be well beyond the diffraction limit. In fact, over 40 pixels would fit inside the Airy disk and well over a billion pixels (one gigapixel) can fit on a single chip. Although it is possible to construct such an imager, contemporary imagers do not take advantage of this possibility. Further, contemporary practice does not contemplate the best ways to implement such a device.
In view of the foregoing, it is beneficial to provide implementations and practical applications for such SDL pixels. More particularly, it is desirable to provide a method for providing a digital film sensor that emulates, at least to some degree, contemporary silver halide film.